optimizer: local known to satisfy predicate => #t in a boolean context

For example, `(if (pair? x) (if x e1 e2) e3)` => `(if (pair? x) e1 e3)`.

pkgs/racket-pkgs/racket-test/tests/racket/optimize.rktl

@@ -1340,6 +1340,15 @@
 (test-comp '(if (if (list? (cons 1 null)) #t #t) 1 2)
            '1)
 
+(test-comp '(let ([x '(7)])
+              (list x x (if (if (list? (cons 1 null)) x 3) 0 1)))
+           '(let ([x '(7)])
+              (list x x 0)))
+(test-comp '(lambda (x)
+              (cons (car x) (if x 1 2)))
+           '(lambda (x)
+              (cons (car x) 1)))
+
 (test-comp '(lambda (y)
               (let ([f (lambda (x) x)])
                 (if f

racket/src/racket/src/optimize.c

@@ -6298,6 +6298,12 @@ Scheme_Object *scheme_optimize_expr(Scheme_Object *expr, Optimize_Info *info, in
         }
       } else if (is_mutated) {
         info->vclock += 1;
+      } else if (context & OPT_CONTEXT_BOOLEAN) {
+        Scheme_Object *pred;
+        pred = optimize_get_predicate(pos, info);
+        if (pred)
+           return scheme_true;
+           /* all predicates recognize non-#f things */
       }
 
       delta = optimize_info_get_shift(info, pos);